The steam generators of pressurized-water nuclear reactors comprise a casing of generally cylindrical shape, within which a water reservoir is delimited by a tube plate of great thickness, on which the bundle of tubes of the steam generator is fixed. Each one of the tubes of the bundle comprises two parallel rectilinear branches, the end parts of which are fixed within the tube plate, generally by rolling in and by welding.
In the steam generator in service, the tubes are in contact both with the primary fluid of the nuclear reactor constituted by pressurized water and with the secondary fluid constituted by feed water which vaporizes. Under the conditions of use of the steam generator, the tubes are liable to suffer various deteriorations and, in particular, corrosion by the fluids with which they are in contact. Certain tubes of the steam generator may become defective in service and exhibit, for example, cracks which give rise to leakages causing contamination of the secondary fluid.
It is therefore necessary, during the periods of maintenance of the reactor, to take action on the steam generator and to plug the tubes exhibiting deteriorations which are liable to involve leakages.
Plugs for obturating the tubes of a steam generator are known which comprise a casing of tubular form, closed at one of its ends by a base and a clamping core, which is likewise tubular, introduced into the bore of the casing of the plug in order to ensure the diametral expansion of the plug after its introduction into the tube of the steam generator.
The external wall of the casing of the plug comprises ribs projecting radially towards the exterior, over at least a part of its length. The clamping by the core is effected, in particular, in the zone of the casing of the plug comprising ribs which cooperate with the internal surface of the tube in order to ensure the blocking and the sealing of the plug.
The core remains in position within the casing of the plug in the course of the restoration to operation of the steam generator after maintenance, and ensures, in particular, the retention of the plug, the base of which is subjected on one side to the pressure prevailing within the tube of the steam generator and on the other side to the pressure of the primary water filling the water reservoir of the steam generator.
In the case of a tube of the steam generator exhibiting a leak, the pressure within the tube corresponds to the pressure of the feed water of the steam generator, this pressure being very much less than the primary pressure.
There is likewise carried out, before the starting up of the steam generators, the preventive plugging of certain tubes which run the risk of cracking rapidly and of producing undesirable leaks. In this case, the internal volume of the tube is filled with air which is isolated both from the primary water and from the feed water, and which is at a pressure very much lower than the primary pressure.
In all cases, the tubes of the steam generator are obturated at each one of their ends which open on either side of the partition of the water reservoir.
The casing of the plug is constructed of a nickel alloy which has undergone a heat treatment permitting the improvement of its resistance to corrosion and mechanical and thermal stresses, within the environment of the steam generator in service.
However, it has been observed that, after a certain time of operation of the steam generator, the casing of the plugs for obturating the tubes of the bundle of which the plugging has been carried out exhibits circumferential cracks, in particular in the zones situated just above and just below the expander core.
These cracks may lead to fractures of the plugs which are reflected in a loss of sealing of the end of the tube, so that the primary water is liable to penetrate within the tube.
Moreover, in the case orf a fracture above the expander core, the upper part of the plug comprising the sealed closure base is propelled violently within the tube under the effect of the difference between the primary pressure and the pressure prevailing within the tube. The upper part of the plug then behaves as a projectile liable to damage the tube of the corresponding steam generator, and the adjacent tubes of the bundle.
Until now, no device was known for limiting the leakages and preventing the risks of fracture and of high-speed ejection of the casings of the plugs for obturating the steam generator tubes of the pressurized-water nuclear reactors.